What are those error codes I see in firebird.log on Linux?

Errors like 'INET/inet_error: read errno = ' are networking errors. For Windows error codes (10060 and similar) read FAQ #119. Here's a list of meanings for Linux error codes:

001 /* Operation not permitted */
002 /* No such file or directory */
003 /* No such process */
004 /* Interrupted system call */
005 /* I/O error */
006 /* No such device or address */
007 /* Argument list too long */
008 /* Exec format error */
009 /* Bad file number */
010 /* No child processes */
011 /* Try again */
012 /* Out of memory */
013 /* Permission denied */
014 /* Bad address */
015 /* Block device required */
016 /* Device or resource busy */
017 /* File exists */
018 /* Cross-device link */
019 /* No such device */
020 /* Not a directory */
021 /* Is a directory */
022 /* Invalid argument */
023 /* File table overflow */
024 /* Too many open files */
025 /* Not a typewriter */
026 /* Text file busy */
027 /* File too large */
028 /* No space left on device */
029 /* Illegal seek */
030 /* Read-only file system */
031 /* Too many links */
032 /* Broken pipe */
033 /* Math argument out of domain of func */
034 /* Math result not representable */
035 /* Resource deadlock would occur */
036 /* File name too long */
037 /* No record locks available */
038 /* Function not implemented */
039 /* Directory not empty */
040 /* Too many symbolic links encountered */
042 /* No message of desired type */
043 /* Identifier removed */
044 /* Channel number out of range */
045 /* Level 2 not synchronized */
046 /* Level 3 halted */
047 /* Level 3 reset */
048 /* Link number out of range */
049 /* Protocol driver not attached */
050 /* No CSI structure available */
051 /* Level 2 halted */
052 /* Invalid exchange */
053 /* Invalid request descriptor */
054 /* Exchange full */
055 /* No anode */
056 /* Invalid request code */
057 /* Invalid slot */
059 /* Bad font file format */
060 /* Device not a stream */
061 /* No data available */
062 /* Timer expired */
063 /* Out of streams resources */
064 /* Machine is not on the network */
065 /* Package not installed */
066 /* Object is remote */
067 /* Link has been severed */
068 /* Advertise error */
069 /* Srmount error */
070 /* Communication error on send */
071 /* Protocol error */
072 /* Multihop attempted */
073 /* RFS specific error */
074 /* Not a data message */
075 /* Value too large for defined data type */
076 /* Name not unique on network */
077 /* File descriptor in bad state */
078 /* Remote address changed */
079 /* Can not access a needed shared library */
080 /* Accessing a corrupted shared library */
081 /* .lib section in a.out corrupted */
082 /* Attempting to link in too many shared libraries */
083 /* Cannot exec a shared library directly */
084 /* Illegal byte sequence */
085 /* Interrupted system call should be restarted */
086 /* Streams pipe error */
087 /* Too many users */
088 /* Socket operation on non-socket */
089 /* Destination address required */
090 /* Message too long */
091 /* Protocol wrong type for socket */
092 /* Protocol not available */
093 /* Protocol not supported */
094 /* Socket type not supported */
095 /* Operation not supported on transport endpoint */
096 /* Protocol family not supported */
097 /* Address family not supported by protocol */
098 /* Address already in use */
099 /* Cannot assign requested address */
100 /* Network is down */
101 /* Network is unreachable */
102 /* Network dropped connection because of reset */
103 /* Software caused connection abort */
104 /* Connection reset by peer */
105 /* No buffer space available */
106 /* Transport endpoint is already connected */
107 /* Transport endpoint is not connected */
108 /* Cannot send after transport endpoint shutdown */
109 /* Too many references: cannot splice */
110 /* Connection timed out */
111 /* Connection refused */
112 /* Host is down */
113 /* No route to host */
114 /* Operation already in progress */
115 /* Operation now in progress */
116 /* Stale NFS file handle */
117 /* Structure needs cleaning */
118 /* Not a XENIX named type file */
119 /* No XENIX semaphores available */
120 /* Is a named type file */
121 /* Remote I/O error */
122 /* Quota exceeded */
123 /* No medium found */
124 /* Wrong medium type */